Fatty acid synthase (FASN) is a 250-270 kDa enzyme that uses endogenous carbon sources, e.g. acetyl-CoA and Malonyl-CoA for the production of fatty acids (FAs), primarily palmitate. FASN has six independent catalytic domains, providing fertile opportunity for drug development. In benign cells and tissues, dietary lipids are predominantly utilized for the production of new lipids and FASN has a minor role in the production of FAs. Increased de novo synthesis of fatty acids is characteristic of tumorigenesis and is closely related to the glycolytic pathway. Tumor cells shift from oxidative to glycolytic metabolism, which feeds excess pyruvate to drive de novo FA synthetic pathway to fulfill the increased lipid requirements for aberrant cellular proliferation. As a consequence, FASN has increased expression and activity in tumor cells that correlates with advanced tumor stage and grade, poor patient prognosis, and disease-free survival.
A number of FASN inhibitors have been developed with a wide array of chemical structures, including compounds with long aliphatic groups, curcuminoids, and polyphenolic compounds. However, these compounds are either in early stages of preclinical development or are limited by severe side-effects.
Alternatively, it was discovered that Orlistat is a particularly effective FASN inhibitor. Orlistat (FIG. 2B) is a lipstatin analog, acts as a lipase inhibitor, and is FDA-approved as a weight loss aid to block the absorption of dietary fat. Crystallographic studies have shown that Orlistat inhibits FASN by directly interacting with the thioesterase domain. The major challenge in the further development of Orlistat as a highly promising chemotherapeutic agent is its high hydrophobicity and poor bioavailability. This results in the need to use extremely large doses to generate a tumor response in mice, which could incur undesirable side effects.
Nanometer-sized particles, approximately 10-100 nm, have functional and structural properties that are not available from either small molecules or from bulk materials. For example, nanoparticles (NP) have a large surface area to volume ratio, which allows the conjugation of tumor-specific targeting ligands (e.g. small molecules, peptides, or antibodies), therapeutics, or diagnostic agents. One recent advancement has been the development of biodegradable NPs with increased therapeutic loading capacity for drug delivery. Thus, development of biocompatible NPs for targeted therapy is an area of considerable interest. While several types of NPs, such as quantum dots, gold, or iron oxide, result in a wealth of properties that can be precisely tuned, these NPs can remain in the body for prolonged times and thus, most of the work related to metallic nanoparticles is still in preclinical development. Consequently, the majority of NPs in clinical trials are based on polymers of liposomes.
As of 2012, six NP drug formulations for cancer therapy are FDA-approved and used in clinics, while more than a dozen more are in clinical use for other diseases and conditions. Due to their mesoscopic size, NPs are preferentially accumulated in tumor stroma due to increased vascular permeability and poor lymphatic drainage out of tumors, which is commonly referred to as the enhanced permeability and retention (EPR) effect. Tumor accumulation can further be increased by conjugation of tumor-specific targeting ligands that can improve intracellular accumulation, drug efficacy, and reduce off-target toxicity. Example targeted, macromolecular FDA-approved therapies, include imatinib for chronic myeloid leukemia and trastuzumab for human epidermal growth factor-2 (HER-2) positive breast cancer.
It is believed that FASN inhibitors could provide an effective means of chemotherapy by stopping production of FAs needed for new cells.
Hyaluronic acid is a ligand for the transmembrane receptor, CD44. Native CD44 along with its various isoforms e.g. CD44v6, can be found to various degrees on benign and malignant cells. CD44 has been shown to be a common marker for tumor progenitor cells as well as cells of colon, head and neck, hepatocellular, non-small cell lung, prostate and breast cancer. HLA is readily catabolized by hyaluronidases (HYAL), primarily HYAL-1 and HYAL-2. Both CD44 and HYAL are upregulated in malignant tissue and HYAL associated with malignant tumors is several times more active, especially in prostate tumors. Thus, HLA renders a NP targeted to CD44 and biodegradable in the tumor microenvironment.
HLA has been conjugated to various hydrophobic ligands such as ceramide, bile acids, or poly [lactide-(co-glycolic acid)] to drive self-assembly into nanoparticles for targeted delivery of chemotherapeutic drugs, such as paclitaxel and doxorubicin. It has been revealed that diminished uptake of HLA-butyric acid-fluorescein conjugates occur when CD44+ MCF-7 cells were pretreated with anti-CD44 MAbJ173 antibody.
Recently, Amiji and colleagues developed a library of HLA (20 kDa) NPs modified with various hydrophobic ligands and cationic moieties for siRNA delivery to lung, breast, liver, and melanoma tumor models. Interestingly, HLA conjugates showed that gene silencing activity in vitro and in vive was linear with CD44 expression.